Fertilizers are responsible for over half of global food production, but there are areas in world with nutrient deficiency and other areas of nutrient excess.
Managing mineral plant nutrients requires careful application of science and skill to meet production, environmental, and social goals.

Tuesday, July 16, 2013

The Roots of Nutrient Uptake

We clearly see that plant growth suffers when there are low
concentrations of nutrients in the soil, when the nutrients are not
sufficiently soluble, or if nutrients do not move to the roots. But in our
quest to grow abundant and healthy crops, it is easy to overlook all of the
complex chemical and biological activity occurring around the plant roots that
make nutrients available for uptake.

Model of root growth

The availability of plant nutrients for roots is controlled by factors such
as soil properties, root characteristics, and interactions with surrounding
microorganisms. Traditional soil testing techniques measure the
availability of nutrients in the general soil, but this may differ from the
nutrient concentration in the immediate vicinity of the root (the rhizosphere).
Nutrients with restricted mobility in the soil (such as P, K, zinc, iron,
manganese, and copper), may be in adequate supply in the bulk soil, but their
concentration may be low near the root if the movement in the soil is too slow
to replenish the nutrients entering the root.

Calcium deficiency results in damaged root tips

Focusing on P as an example, supplying this nutrient to the root includes
several complicated mechanisms. This involves excretion of organic acids,
increased root hair formation, and enzyme release.

• Release of Organic Acids: When
soil P supplies are low, many plants excrete a wide range of organic compounds
to increase the availability of relatively insoluble compounds, such as some
calcium phosphate minerals. The organic acids have a role in dissolving
nutrients (due to pH), complexing soil cations, and providing an excellent
growth substrate for soil microorganisms. Common organic exudates include
substances such as malate, citrate, acetate, and oxalate which can lead to
root-zone modification. Most soils have populations of microorganisms that are
capable of dissolving P-containing minerals, so addition of an organic
substrate may encourage their growth in low-P conditions. Mycorrhizal fungi
also form complex relationships with most plant species, where the fungi
provide various benefits for the plant, including improved nutrition, in
exchange for carbohydrate for fungal maintenance and growth.

Rhizosphere soil on wheat roots

• Changes in Root Structure: Plants growing in a low-P soil tend to direct more
of their photosynthate energy to root development and often have more fine
roots with a small diameter, resulting in a larger surface area. A large root
surface area allows plants to access more of the soil and scavenge any soluble
phosphate that may be present.
• Enzyme Release: In low-P conditions, plants generally increase the production
of enzymes that enhance the rate of P release from soil organic matter. The
phosphatase enzymes are not effective in mineralizing phytate, the dominant
form of organic P in many soils. Phytase, the enzyme responsible for phytate
hydrolysis, is primarily released by microorganisms, which indirectly improves
the P availability for nearby roots.

Root hairs increase surface area

These root modifications occur as a result of low soil P
availability, requiring plants to devote additional energy to the roots and
away from above-ground growth. The excretion of organic compounds
from roots can consume as much as half of all the carbon allocated to the root
system, although this number is highly variable. The energy costs of
mycorrhizal associations with various plant species ranges from 4 to 20% of the
daily net photosynthesis. Plant geneticists are looking for ways to make plant
roots more efficient at recovering nutrients from the soil. While we wait for
improved plant genetics, there are many practical things that can be done to
get the maximum benefit from added nutrients. Begin by placing nutrients in the
soil in the proper form and in the correct place so plant roots can support
abundant yields of high-quality products.

Cross section of rice roots (notice the aerenchyma, the air channel that allows gas exchange between shoot and root)

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About Me

I am a soil scientist with interest in managing plant nutrients in the best way possible. I am fortunate to be able to work in research and education to be able to accomplish this goal.
After receiving a PhD in Soil Science at the University of California (Riverside), I worked as a Research Scientist for the U.S. government, as a Professor of Soil Science, and now I work for a not-for-profit institution. It's been a wonderful experience!